UBSAN|downcast of address X which does not point to an object of type 'Item_string' in sql/json_schema.cc

SET @schema='{ "type":"object","patternProperties": { "^I_": {"type":"number"},"^S_" : {"type":"string"} } }';

SET SESSION sql_mode='empty_string_is_null';

SELECT JSON_SCHEMA_VALID (@schema,'{"key1":"val0","key2":0,"I_int":0,"S_":"abc","prop0":"str0"}');

/test/JSON/11.1_dbg_san/sql/json_schema.cc:2200:86: runtime error: downcast of address 0x6290000d9128 which does not point to an object of type 'Item_string'

0x6290000d9128: note: object is of type 'Item_null'

 00 00 00 00  88 ad 93 07 50 56 00 00  00 00 00 00 00 00 00 00  20 be bb 0f 50 56 00 00  06 00 00 00

              ^~~~~~~~~~~~~~~~~~~~~~~

              vptr for 'Item_null'

    #0 0x56500309d6a7 in Json_schema_pattern_properties::handle_keyword(THD*, st_json_engine_t*, char const*, char const*, List<Json_schema_keyword>*) /test/JSON/11.1_dbg_san/sql/json_schema.cc:2200

    #1 0x56500309ac6b in create_object_and_handle_keyword(THD*, st_json_engine_t*, List<Json_schema_keyword>*, List<Json_schema_keyword>*) /test/JSON/11.1_dbg_san/sql/json_schema.cc:2759

Compiled with GCC 9.4.0

-DWITH_UBSAN=ON -DCMAKE_CXX_FLAGS=-static-libasan